Polymerizable acrylic acid and other acrylate monomers undesirably polymerize during various stages of the manufacturing, processing, handling, storage and use thereof. One especially troublesome problem is the polymerization of acrylic acid monomer in the purification stages of monomer production. It is well known that the monomers readily polymerize and that such polymerization increases with concurrent increases in temperature.
Common industrial methods for producing acrylic acids include a variety of purification processes, including distillation, to remove impurities. Unfortunately, purification operations carried out at elevated temperatures result in an increased rate of undesired polymerization. Polymerization during the monomer purification process results not only in the loss of desired monomer end-product, but also in the loss of production efficiency caused by polymer formation or agglomeration on process equipment. In heat requiring operations, such agglomeration adversely affects heat transfer and separation efficiency.
A variety of compositions and methods have been proposed for inhibiting uncontrolled polymerization of acrylic acid and esters. Known inhibitors include phenothiazine, methylene blue, hydroquinone, hydroquinone methyl ether (MEHQ), copper compounds, 4-hydroxy-TEMPO and sundry manganese containing compounds.
In U.S. Pat. No. 4,797,504, hydroxylamines and phenylenediamines, in combination, are taught as being effective anti-polymerization aids for acrylate monomer systems. This particular combination has also been shown to be effective in inhibiting acrylonitrile polymerization in U.S. Pat. No. 4,720,566.
Japanese patent publication 50098211 teaches that polymerization of α, β unsaturated carboxylic acids can be inhibited by the use of sundry manganese salts, with or without hydroquinone or MEHQ, save for the particular sodium manganese ethylene diamine tetra-acetate salt. Similarly in U.S. Pat. No. 4,507,495, acrylic acid polymerization is inhibited in ethyl acrylate production methods by the use of manganese or cerium salts that are soluble in the reaction mixture. Manganese nitrite, Mn(NO2)2, is used as a polymerization inhibitor in U.S. Pat. No. 4,663,480.
Of somewhat lesser importance is U.S. Pat. No. 4,638,079 which discloses processes for inhibiting polymerization of polymerizable ethylenically unsaturated monomers such as hydrocarbons, acids and esters wherein a cobalt (III), nickel (II), or manganese (II) complex of N-nitrosophenylhydroxylamine is utilized. In a preferred embodiment of that patent, polymerization of an acrylic acid or acrylate ester is inhibited.
It is desirable to provide a low volatility anti-polymerization treatment so that the treatment will not be carried overhead with the purified monomer during the distillation (i.e., purification) thereof. Some plants will take crude acrylic acid from the bottom of a tower and transfer it to a storage tank, prior to further purification to technical grade, glacial or to esterification.
There has been a specific need for an acrylate anti-polymerization treatment that is readily soluble in non-polar organic solvents, such as xylene or heavy aromatic naphtha, so that the treatment can be dissolved therein and provide a stable product that can be shipped and stored without undue fear of product deterioration and separation. Related treatments are found in, e.g., U.S. Pat. No. 5,171,888.
As noted above, the commercial purification of acrylic acid entails multiple separation steps that result in the formation of unwanted polyacrylic acid in such quantities that the distillation process must be terminated and the distillation tower cleaned of the unwanted polymer. Polymerization inhibitors are continuously injected, but polymer still builds up on distillation trays and in column reboilers.
Polyalkenyl succinimide dispersants, e.g., various polyisobutenyl succinimide (PIBSI) dispersants of relatively high molecular polyisobutenyl weight have been used to disperse polymer into extraction solvents, and are able to mitigate fouling where there are high concentrations of solvent, and low concentrations of acrylic acid. These same PIBSI dispersants have not been successful in locations where there is little to no solvent and high concentrations of acrylic acid, because these dispersants are not soluble in the acrylic acid monomer. It would be desirable to find a succinimide dispersant that would be soluble in acrylic acid and acrylate esters.
These and other needs in the art are addressed by the methods and compositions detailed herein.